Cracking the Circadian Code

The most recent WEEKLY described the molecular basis of circadian rhythm, the internal 24-hour clock that governs many of our biological processes. Disrupting this cycle over the long term is linked to diseases including obesity, diabetes, heart disease—even cancer. Now we turn our focus on some daily habits that can profoundly influence our internal clocks and therefore our health.

Time to Eat

To maintain a healthy weight, science tells us to watch how many calories we take in. Of course, it’s critical to think about the content of those calories too—unfortunately a half pound of hot fudge sundae does not equal a half pound of chicken breast and steamed broccoli. Interestingly, the latest research also suggests that when we consume those calories matters too.

Scientists at the Salk Institute for Biological Studies (San Diego, CA) took two groups of genetically identical mice and gave them access to the same high-fat foods. Sadly for one group though, they could enjoy the chow only during an eight-hour time period.

All of the rodents ate the same amount but the similarity ends there. Like many of us in the U.S., the 24-hour access group “grazed,” munching small amounts of food regularly.  The other mice, meanwhile, ate fewer but larger meals within the eight-hour window. Remarkably, the “eight-hour” mice didn’t gain excess weight or experience unhealthy blood glucose or blood cholesterol levels. Their “24-hour” siblings did.

Follow-up studies also suggest benefit from limiting when we eat to nine, ten, or even 12 hours. These longer windows may prove easier for people to handle. The Salk researchers think that the effort may be worth it. One small study focused on a group of ten overweight people (BMI of greater than 25). They were told to consume all calories— including drinks and snacks!—within a daily ten-hour window. Happily for them, they all lost a significant amount of weight after four months without changing the amount or type of food they ate.

According to Dr. Satchin Panda, who is director of the lab involved, the participants’ success is linked to circadian rhythms. Like the rest of our body, the digestive system works on a 24-hour clock. Prime fat-burning starts six to eight hours after our last meal and continues until we break the “fast.”  Establishing a regular, time-restricted pattern of eating may optimize our digestion, nutrient absorption, and fat-burning.

Eating on schedule may also promote a healthy gut microbiome. Researchers at the Weizmann Institute of Science (Rehovot, Israel) have shown that the microbiome composition of people who are jet-lagged starts to resemble that of obese people. This alteration suggests that messing with circadian rhythms messes with the microbiome of shift workers and travelers who regularly change time zones. Further research from Panda’s lab suggests that sticking to a time-restricted eating schedule may offset this disruption. (Article continues below)

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Time to Exercise


Nutrition and the timing of our meals constitute only part of what’s needed for maintaining a healthy weight. Exercise is another critical component. Could circadian rhythms also influence the benefits of exercise?

Studies suggest that exercising first thing in the morning or in late afternoon best enhances its positive effects. Early morning workouts take advantage of early morning daylight exposure (or bright lights at the gym). They offer a direct way to synch the brain’s “suprachiasmatic nucleus,” the master clock that governs the cells in the rest of your body.

Late afternoon exercise corresponds to the natural circadian peak in muscle tone, or at-rest muscle contraction. Thus people may find high-intensity exercise or weight-training easier in the late afternoon than at other times. In general, exercise physiologists counsel against intense exercise close to bedtime because it raises levels of the stress hormone cortisol, which can interfere with sleep.

Moderate activity, such as walking after dinner, is another story. It may help regulate blood sugar levels. This is because insulin levels and glucose tolerance drop nightly as part of a 24-hour cycle. Light to moderate exercise also causes muscle cells to absorb glucose from the blood stream, compensating for reduced insulin levels.

Regular sleep deprivation, whether from shift work, jet setting or simply not taking care of ourselves can do more than make us a little cranky—it can damage our health in serious ways. Maintaining a regular schedule for sleeping, eating, and exercise can help to ensure that we work with our circadian clocks to obtain optimal health. For even more information about this topic, Dr. Satchin Panda’s new book, The Circadian Code, delves more deeply into the science behind time-restricted eating and the interaction of our circadian rhythms and our overall health. 

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